When external pressure is applied to an optical fibre, the core of the fibre experiences strain which varies with the applied pressure. Due to a phenomenon known as the “elasto-optic effect”, the way in which light propagates in the fibre's core changes as the strain experienced by the core changes. So, by looking at the way in which the light propagation properties of the fibre's core change, changes in strain and hence pressure applied to the fibre can measured.
Most conventional optical fibres have circular symmetry in cross section and, indeed, are uniformly cylindrical. This means that when external isotropic pressure is applied to them, e.g. when they are immersed in fluid, mechanical stress of substantially the same magnitude in every direction orthogonal to the length of the fibre is applied to the fibre's core. Any deformation of the core is therefore substantially uniform orthogonal to its length. In addition, the silica from which optical fibres are usually made is relatively incompressible. The strain experienced by the fibre's core is therefore relatively small for a given pressure and the optical properties of standard optical fibres only vary very slightly with changes in external isotropic pressure. This insensitivity makes them fairly impractical for pressure measurement.
It has therefore been suggested to use optical fibres adapted to experience asymmetrical strain at their cores under the influence of isotropic external pressure for measuring pressure in fluids. One such fibre is known as a side-hole fibre. A side-hole fibre typically has two air holes that extend parallel to the core along the length of the fibre, the holes being positioned on either side of the core. The presence of the air holes means that the core of the fibre experiences less stress in a direction transverse to its length that extends between the air holes than in other directions transverse to its length when isotropic external pressure is applied to the fibre. Indeed, the direction of greatest stress is that orthogonal to a line extending between the air holes. The core therefore experiences greater strain in this direction than others. One consequence of the asymmetrical strain experienced by side-hole fibres is that the refractive index of the fibre's core changes more for light linearly polarised in the direction of greatest strain than for light linearly polarised in the orthogonal direction. In other words, the fibre's birefringence changes, with the so-called fast axis in the direction of greatest strain. The change in birefringence is proportional to the applied external isotropic pressure and large in comparison to changes in optical properties of standard optical fibres. Side-hole fibres are therefore much more useful for pressure measurement than standard fibres.
Changes in fibre birefringence can be measured in a variety of ways. One well established method involves use of a Bragg grating written in the fibre, often referred to as a Fibre Bragg Grating (FBG). FBGs are described, for example, in the paper “Fiber Bragg Grating Technology Fundamentals and Overview”, Kenneth O. Hill et al, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997. Briefly, an FBG comprises a periodic modulation in the isotropic refractive index of a fibre's core along the length of the core. This modulation can be written into the core using interfering coherent light, e.g. using an ultra violet (UV) laser and an appropriate optical arrangement to create a standing wave in the fibre that selectively heats the fibre's core to change its refractive index at the desired locations to create a grating. When light travels along an FBG, some of it is reflected, with the reflection occurring most strongly at a wavelength of light known as the Bragg wavelength λB, which can be expressed asλB=2nΛB where n is the refractive index of the fibre and ΛB is the period of the grating. Changes in refractive index of the fibre therefore cause proportional changes in the Bragg wavelength λB. Indeed, a change in the birefringence of a side-hole fibre at the FBG results in there being effectively two Bragg wavelengths, with one wavelength being more sensitive to changes in applied external isotropic pressure than the other. This allows measurement of applied external isotropic pressure and some compensation for other influences on fibre birefringence, such as temperature. This is described in more detail, for example, in the paper “Thermally Insensitive Pressure Measurements up to 300° C. Using Fiber Bragg Gratings Written on to Side Hole Single Mode Fiber”, Tsutomu Yamate et al, SPIE Proceedings, Vol. 4185, p 628, 2000.
Another method involves the use of a polarisation rocking filter. This is described, for example, in the paper “Approach to Highly Sensitive Pressure Measurements Using Side-Hole Fibre”, J. A. Croucher et al, Electronics Letters, Vol. 34, No. 2, 22 Jan. 1998. Briefly, unlike an FBG, which comprises a periodic modulation of a fibre core's isotropic refractive index, a rocking filter comprises a periodic modulation of the core's birefringence. A rocking filter converts light travelling along the filter between two orthogonal polarisation states. The wavelength λR at which this conversion occurs most strongly, i.e. the resonant wavelength, can be expressed asλR=bΛR where b is the fibre birefringence and ΛB is the pitch of the grating. So, as the birefringence b of a side hole fibre in which a rocking filter is written changes, the wavelength λR of light most strongly converted from one polarisation state to the orthogonal polarisation state as it passes through the rocking filter changes proportionally. Again, this allows measurement of external isotropic pressure applied to the fibre. It has been found that the use rocking filters can allow measurements up to 80,000 times more sensitive to changes in applied pressure than measurements using FBGs.
However, the use of side-hole fibres incorporating either FBGs or rocking filters has a number of problems. In particular, pressure can only be measured at the location of the FBG or rocking filter. This means that the measurement is restricted by the size and location of the FBG or rocking filter along the length of the fibre. This significantly limits the usefulness of optical fibres for pressure measurement.
For example, an FBG typically has a pitch ΛB below 1 μm and light is strongly reflected by an FBG of only a few cm in length. So, changes in applied external pressure measured using an FBG only relate to the pressure applied to the fibre over a few cm of its length. Furthermore, it is very difficult to measure the pressure applied to a fibre at multiple positions along its length using FBGs, as the provision and use of multiple FBGs in a single optical fibre is limited and complex. For example, in order to distinguish between different FBGs, the different FBGs need to be addressed using light of slightly different wavelengths, say around 2 nm apart from one another. As it is only possible to interrogate the fibre with light having a limited wavelength range, say around 60 nm, the number of different FBGs that can be used in a single fibre is severely limited, say to around 30. This can be mitigated to some extent by looking at light reflected at different times from a light pulse travelling along the fibre to distinguish between light reflected at different FBGs. In other words, some wavelength re-use can be achieved using time division multiplexing. However, it is impossible to avoid totally influence by other FBGs on the light reflected in one FBG. In other words, cross talk between the FBGs inevitably occurs. Even using time division multiplexing, the maximum likely number of FBGs that could be used in a single fibre is therefore around 100.
Similar problems occur with rocking filters. The pitch λR of a typical rocking filter written in a side-hole fibre is longer (e.g. at least 2000 times longer) than that of an FBG and can range from a few millimeters to a few meters. This means that measurements of applied pressure using a single rocking filter generally relate to the average pressure over a considerable length of the fibre. It is also difficult to measure the pressure applied to a fibre at multiple positions along its length as the provision and use of multiple rocking filters in a single fibre is limited and complex. Like FBGs, the number of rocking filters that can be used in a single fibre is limited by wavelength restrictions and crosstalk problems, with again only a very few rocking filters being useable in a single fibre.
Another problem is that the fabrication of side-hole fibres incorporating FBGs or rocking filters is relatively complex and expensive. More specifically, writing FBGs and rocking filters in optical fibres is a difficult and time consuming process. Sensor heads incorporating FBG and rocking filter fibres can therefore be relatively expensive. This is a particular problem when the sensor heads are used in harsh or dangerous environments in which they are likely to be damaged.
The present invention seeks to overcome these problems.